The viscosity-temperature relationship of a lubricating oil is one of the critical criteria which must be considered when selecting a lubricant for a particular application. Viscosity index (VI) is an empirical, unitless number which indicates the rate of change in the viscosity of an oil within a given temperature range and is related to kinematic viscosities measured at 40° C. and 100° C. (typically using ASTM Method D445). Fluids exhibiting a relatively large change in viscosity with temperature are said to have a low viscosity index. A low VI oil, for example, will thin out at elevated temperatures faster than a high VI oil. Usually, the high VI oil is more desirable because it has higher viscosity at higher temperature, which translates into better or thicker lubrication film and better protection of the contacting machine elements. In another aspect, as the oil operating temperature decreases, the viscosity of a high VI oil will not increase as much as the viscosity of a low VI oil. This is advantageous because the excessive high viscosity of the low VI oil will decrease the efficiency of the operating machine. Thus high VI (HVI) oil has performance advantages in both high and low temperature operation. VI is determined according to ASTM method D2270.
Polyalphaolefins (PAOs) comprise a class of hydrocarbons manufactured by the catalytic oligomerization (polymerization to low molecular weight products) of linear alpha-olefins (LAOS) typically ranging from 1-hexene to 1-octadecene, more typically from 1-octene to 1-dodecene, with 1-decene as the most common and often preferred material. Such fluids are described, for example, in U.S. Pat. No. 6,824,671 and patents referenced therein.
Polyalphaolefins produced by conventional Friedel-Crafts catalysts, however are usually characterized by having extra relatively short branches, such as methyl and ethyl short side chains, even though the feed olefins do not contain these short branches. This is thought to be because Friedel-Crafts catalysts partially isomerize the starting alpha-olefins and the intermediates formed during the oligomerization process. The presence of short chain branches typically is less desirable for superior lubricant properties, including VI and volatility.
High viscosity index polyalpha-olefin (HVI-PAO) prepared by, for instance, polymerization of alpha-olefins using reduced metal oxide catalysts (e.g., chromium) are described, for instance, in U.S. Pat. Nos. 4,827,064; 4,827,073; 4,990,771; 5,012,020; and 5,264,642. These HVI-PAOs are characterized by having a high viscosity index of 130 and above, a branch ratio of less than 0.19, a weight average molecular weight (Mw) of between 300 and 45,000, a number average molecular weight (Mn) of between 300 and 18,000, a molecular weight distribution (MWD=Mw/Mn) of between 1 and 5, and pour point below −15° C. Measured in carbon number, these molecules typically range from C30 to C1300.
In the production of PAOs and HVI-PAOs, the feed may be limited to one specific alpha-olefin, usually 1-decene. Occasionally, when 1-decene is not available in large enough quantity, small to moderate amounts of 1-octene or 1-dodecene are added to make up the quantity. When mixtures of feed are used, the products tend to be blocky copolymers rather than random copolymers and/or products produced at the beginning of the process are different than that produced at the end of the process, and the inhomogeneous polymer product will be characterized by poor viscosity indices and poor low temperature properties. Thus, in the past, PAOs and HVI-PAOs have typically been made using pure C10 feeds. Although, U.S. Pat. No. 7,547,811 discloses mixed feed PAO's made using AlCl3 type catalysts.
One successful example of utilizing mixed feed alpha-olefins to produce HVI-PAOs is the process disclosed in WO 2007/011462 which discloses an improved process wherein mixed alpha-olefin feedstocks are polymerized over an activated metallocene catalyst to provide essentially random liquid polymers particularly useful in lubricant components or as functional fluids. The activated metallocene catalyst can be simple metallocenes, substituted metallocenes or bridged metallocene catalysts activated or promoted by, for instance MAO or a non-coordinating anion.
One problem facing producers of HVI-PAOs is that of reducing the unsaturation of the as-polymerized carbon chains of the PAO products, which can be quantified by Bromine number (ASTM D1159). A PAO fluid cannot be satisfactorily used as a lubricant basestock if its Bromine number exceeds 2. The unsaturation indicated by higher Bromine number can result in poor high temperature stability of the PAO molecules. Accordingly, it is typical to hydrogenate these as-polymerized PAO products in order to reduce the level of unsaturation in the molecules, so as to render them suitable for use as lubricant basestocks. WO 2007/011462 discloses post-oligomerization hydrogenation in order to produce a polyalphaolefin having a Bromine number of less than 1.8.
However, it has been suggested that the oligomerization reaction can be conducted in the presence of low levels of hydrogen, so as to improve catalyst productivity (see, for example, WO 2007/011462 at paragraph [0115]).
U.S. Pat. No. 6,858,767 discloses a process for producing liquid PAO polymer by contacting 1-decene with a particular type of metallocene catalyst, activated with an alkylaluminoxane, in the presence of hydrogen. The resulting product is disclosed to possess a unique combination of properties, such as low molecular weight, low polydispersity index, controllable kinematic viscosity, low Iodine Number and low glass transition temperature. The resulting product is disclosed to be suitable as a viscosity modifier.
Efforts have been made to prepare various PAOs using metallocene catalyst systems. Examples include U.S. Pat. No. 6,706,828 (equivalent to US 2004/0147693), where PAOs are produced from meso-forms of certain metallocene catalysts under high hydrogen pressure. Comparative example D of U.S. Pat. No. 6,706,828, however, uses rac-dimethylsilylbis(2-methyl-indenyl)zirconium dichloride in combination with methylalumoxane (MAO) at 100° C. in the presence of hydrogen to produce polydecene having a reported Kinematic Viscosity at 100° C. (KV100) of 116 cSt, a Kinematic Viscosity at 40° C. (KW) of 1039 cSt, a VI of 214, an iodine number of 2.8, an Mw of 7084, an Mn of 2906, an Mw/Mn of 2.4, and a Tg of −72.4° C. Likewise, WO 02/14384 discloses, among other things, in examples J and K the use of rac-ethyl-bis(indenyl)zirconium dichloride or rac-dimethylsilyl-bis(2-methyl-indenyl) zirconium dichloride in combination with MAO at 40° C. (at 200 psi hydrogen or 1 mole of hydrogen) to produce isotactic polydecene reportedly having a Tg of −73.8° C., a KV100 of 702 cSt, and a VI of 296; or to produce polydecene reportedly having a Tg of −66° C., a KV100 of 1624, and a VI of 341, respectively. Further WO 99/67347 discloses in example 1 the use of ethylidene bis(tetrahydroindenyl)zirconium dichloride in combination with MAO at 50° C. to produce a polydecene having an Mn of 11,400 and 94% vinylidene double bond content.
Others have made various PAOs, such as polydecene, using various metallocene catalysts not typically known to produce polymers or oligomers with any specific tacticity. Examples include WO 96/23751, EP 0 613 873, U.S. Pat. No. 5,688,887, U.S. Pat. No. 6,043,401, WO 03/020856 (equivalent to US 2003/0055184), U.S. Pat. No. 5,087,788, U.S. Pat. No. 6,414,090, U.S. Pat. No. 6,414,091, U.S. Pat. No. 4,704,491, U.S. Pat. No. 6,133,209, and U.S. Pat. No. 6,713,438.
U.S. Pat. No. 6,548,724 (equivalent to US 2001/0041817 and U.S. Pat. No. 6,548,723) disclose production of oligomer oils using certain metallocene catalysts, typically in combination with methylalumoxane. Column, 20, line 40 to 44 of U.S. Pat. No. 6,548,724 indicates that Examples, 10-11 indicate that di, tri or tetra substitutions on the cyclopentadienyl rings of the metallocenes are useful for production of high viscosity polyalphaolefins, (viscosities in the range of 20 to 5000 cSt at 100° C.) with improved yields whereas penta alkyl substituted cyclopentadienyl rings are reported as poor.
WO 2007/011459 describes the production of isotactic polyalphaolefins from monomers having 5 to 24 carbon atoms using racemic metallocenes and non-coordinating anion activators.
WO 2007/011973 discloses a process to produce lower viscosity, higher Bromine number polyalphaolefins in the presence of an unbridged substituted metallocene catalyst, a non-coordinating anion activator, and optional hydrogen.
WO 2008/010865 discloses a process to produce high viscosity, atactic PAO fluids in the presence of a metallocene catalyst, a non-coordinating anion activator, and hydrogen.
WO 2009/017953 discloses a process to produce liquid, atactic poly-alphaolefin in the presence of a meso-metallocene catalyst with a non-coordinating anion activator.
WO 2009/137264 and US 2009/0281360 disclose a process to produce a PAO composition having from 0.5 to 5 mole % of mm triads and from 40 to 58 mole % of rr triads, and preferably having from 37 to 59.5 mole % of mr triads. The PAO composition ideally is substantially free of peaks in a region of from 27.0 to 29.0 ppm, and/or in a region of 20.0 ppm and/or in a region of 42.5 ppm in a 13C NMR spectrum. The PAO composition preferably has a high degree of saturation, and ideally has an Iodine Number of from 0.2 to 5. The PAO composition preferably is formed by polymerizing an olefin monomer, e.g., a C8-C12 olefin, preferably 1-decene, in the presence of a metallocene catalyst, preferably a bridged metallocene, and hydrogen.
Other references of interest include: U.S. Pat. No. 7,129,197, U.S. Pat. No. 5,177,276, U.S. Pat. No. 5,731,254, U.S. Pat. No. 4,892,851, U.S. Pat. No. 6,706,828, EP0284708, U.S. Pat. No. 5,846,896, U.S. Pat. No. 5,679,812, EP0321852, U.S. Pat. No. 4,962,262, EP0513380, US2004/0230016, and U.S. Pat. No. 6,642,169.
To date however, PAO's made with metallocenes have not found wide applicability in the marketplace, particularly the lubricant marketplace, due to inefficient process, cost and property deficits. The instant disclosure address such and other needs by providing new PAO's and or HVI-PAO's having excellent property combinations and an improved process to produce them.
Further, despite recent advances, there remains an unmet need in the art to optimize the polymerization reaction process for producing PAOs, so as to avoid the need for expensive, post-polymerization hydrogen finishing, such that the as-polymerized product is suitable for use as a lubricant basestock. Also, there is a need to improve catalyst productivity, so that the cost for the total catalyst system can be reduced and the catalyst system removal can be simplified. This improved productivity can improve the overall process economics.